Method of forming a SiGe DIAC ESD protection structure

ABSTRACT

A diode for alternating current (DIAC) electrostatic discharge (ESD) protection circuit is formed in a silicon germanium (SiGe) hetrojunction bipolar transistor (HBT) process that utilizes a very thin collector region. ESD protection for a pair of to-be-protected pads is provided by utilizing the base structures and the emitter structures of the SiGe transistors.

This is a divisional application of application Ser. No. 11/890,097filed on Aug. 2, 2007 now U.S. Pat. No. 7,514,751 by VladislavVashchenko et al.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a DIAC ESD protection structure and,more particularly, to a method of forming a SiGe DIAC ESD protectionstructure.

2. Description of the Related Art

A diode for alternating current (DIAC) is a bidirectional diode that iscommonly used in alternating current (AC) applications. In operation,when the voltage across a DIAC is less than a breakdown voltage, theDIAC is substantially non-conductive, providing a high-resistancecurrent path between two nodes.

However, when the voltage across the DIAC exceeds the breakdown voltage,the DIAC becomes conductive, providing a low-resistance current pathbetween the two nodes. The DIAC continues to provide a low-resistancecurrent path until the current flowing through the DIAC falls below aholding current, at which time the DIAC switches back and again providesa high-resistance current path. Because of these operationalcharacteristics, DIAC structures are also used to provide electrostaticdischarge (ESD) protection for semiconductor devices.

FIG. 1 shows a cross-sectional view that illustrates a prior-art CMOSDIAC ESD protection structure 100. As shown in FIG. 1, structure 100includes a p-substrate 110, and a deep n-well 112 that is formed inp-substrate 110. Structure 100 also includes a pair of spaced-apartp-wells 114 and 116 that are formed in deep n-well 112, and a p-well 118that is formed in substrate 110 to lie adjacent to deep n-well 112.Further, structure 100 includes an n+ region 120 that is formed in deepn-well 112 and the p-wells 114 and 116.

In addition, CMOS DIAC ESD protection structure 100 includes an n+region 122 and a p+ region 124 that are formed in p-well 114, an n+region 126 and a p+ region 128 that are formed in p-well 116, and a p+region 130 that is formed in p-well 118. N+ region 122, p+ region 124,and p+ region 130 are connected to a ground pad, while n+ region 126 andp+ region 128 are connected to a to-be-protected pad.

During normal operation, when a positive voltage less than the breakdownvoltage is placed on the to-be-protected pad, the positive voltage isalso present on p+ region 128 and p-well 116. The positive voltage onp-well 116 forward biases the deep n-well 112/n+ region 120 junction,thereby causing holes to be injected into deep n-well 112/n+ region 120.The injected holes raise the potential of deep n-well 112/n+ region 120,thereby reverse biasing the junction between deep n-well 112/n+ region120 and p-well 114. The reverse-biased junction blocks charge carriersfrom flowing from the to-be-protected pad to the ground pad.

In response to an ESD event, however, the reverse-biased junctionbetween deep n-well 112/n+ region 120 and p-well 114 breaks down due toavalanche multiplication. The breakdown of the junction causes holes tobe injected into p-well 114, and electrons to be injected into deepn-well 112. The holes injected into p-well 114 flow over and arecollected by p+ region 124.

In addition, the flow of holes increases the potential of p-well 114 inthe region that lies adjacent to n+ region 122, thereby forward biasingthe junction between p-well 114 and n+ region 122. As a result, p-well114 also injects holes into n+ region 122, while n+ region 122 injectselectrons into p-well 114. Some of the electrons injected into p-well114 drift over and are then injected into deep n-well 112/n+ region 120across the broken down junction. The electrons injected into n-well112/n+ region 120 are swept into p-well 116 across the forward-biasedjunction.

FIG. 2 shows a cross-sectional view that illustrates a prior-art CMOSDIAC ESD protection structure 200. As shown in FIG. 2, structure 200includes a p-substrate 210, and a deep n-well 212 that is formed inp-substrate 210. Structure 200 also includes a p-well 214 that is formedin deep n-well 212, a p-well 218 that is formed in substrate 210 to lieadjacent to deep n-well 212, and an n+ region 220 that is formed in deepn-well 212, p-well 214, and p-well 218.

In addition, CMOS DIAC ESD protection structure 200 includes an n+region 222 and a p+ region 224 that are formed in p-well 218, and an n+region 226 and a p+ region 228 that are formed in p-well 214. N+ region222 and p+ region 224 are connected to a ground pad, while n+ region 226and p+ region 228 are connected to a to-be-protected pad.

During normal operation, when a positive voltage less than the breakdownvoltage is placed on the to-be-protected pad, the positive voltage isalso placed on p+ region 228 and p-well 214. The positive voltage onp-well 214 forward biases the deep n-well 212/n+ region 220 junction,thereby causing holes to be injected into deep n-well 212/n+ region 220.The injected holes raise the potential of deep n-well 212/n+ region 220,thereby reverse biasing the junction between deep n-well 212/n+ region220 and p-substrate 210/p-well 218. The reverse-biased junction blockscharge carriers from flowing from the to-be-protected pad to the groundpad.

In response to an ESD event, however, the reverse-biased junctionbetween deep n-well 212/n+ region 220 and p-substrate 210/p-well 218breaks down due to avalanche multiplication. The breakdown of thejunction causes holes to be injected into p-substrate 210/p-well 218,and electrons to be injected into deep n-well 212/n+ region 220. Theholes injected into p-well 218 flow over and are collected by p+ region224.

In addition, the flow of holes increases the potential of p-well 218 inthe region that lies adjacent to n+ region 222, thereby forward biasingthe junction between p-well 218 and n+ region 222. As a result, holesare also injected into n+ region 222 from p-well 218, while n+ region222 injects electrons into p-well 218. Some of the electrons injectedinto p-well 218 drift over and are injected into deep n-well 212/n+region 220 across the broken down junction. The electrons injected inton-well 212/n+ region 220 are swept into p-well 214 across theforward-biased junction.

FIG. 3 shows a cross-sectional view that illustrates a prior-art silicongermanium (SiGe) hetrojunction bipolar transistor (HBT) 300. As shown inFIG. 3, transistor 300 includes a semiconductor structure 308 that has ap-substrate 310, and an n+ buried layer 312 that touches and lies overp-substrate 310. In addition, semiconductor structure 308 includes ann-type collector region 314 that touches the top surface of n+ buriedlayer 312, an n+ collector region 316 that extends down from the topsurface of semiconductor structure 308 to touch n+ buried layer 312, anda number of shallow trench isolation regions 318 that extend down fromthe top surface of semiconductor structure 308.

In addition, transistor 300 includes a p-type single-crystal-silicongermanium-carbon base region 320 that touches the top surface of n-typecollector region 314, and a p+ polysilicon germanium-carbon base contactregion 322 that touches the side of single-crystal-silicongermanium-carbon base region 320. Transistor 300 also has a silicidelayer 324 that touches the top surface of region 322, and a metal basecontact 326 that touches silicide layer 324.

As further shown in FIG. 3, transistor 300 includes an n+ polysiliconemitter region 330 that touches the top surface ofsingle-crystal-silicon germanium-carbon base region 320, and an n+emitter region 332 that lies in single-crystal-silicon germanium-carbonbase region 320. (N+ emitter region 332 results from the out diffusionof dopants from n+ emitter region 330 during fabrication.) Transistor300 additionally includes an isolation region 340 that isolates baseregion 322 from emitter region 330, a silicide layer 342 that touchesthe top surface of region 330, and a metal emitter contact 344 thattouches silicide layer 342. Transistor 300 operates in a conventionalmanner.

One problem with transistor 300 is that semiconductor structure 308,which has a very thin collector region (314), is incompatible with theCMOS DIAC ESD protection structures 100 and 200, which utilize p-wellsand deep n-wells. As a result, there is a need for a DIAC ESD protectionstructure that is compatible with SiGe HBTS.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a prior-art CMOS DIAC ESDprotection structure 100.

FIG. 2 is a cross-sectional view illustrating a prior-art CMOS DIAC ESDprotection structure 200.

FIG. 3 is a cross-sectional view illustrating a prior-art silicongermanium (SiGe) hetrojunction bipolar transistor (HBT) 300.

FIG. 4 is a cross-sectional view illustrating an example of a SiGe DIACESD protection structure 400 in accordance with the present invention.

FIG. 5 is a cross-sectional view illustrating an example of a SiGe DIACESD protection structure 500 in accordance with the present invention.

FIG. 6 is a cross-sectional view illustrating an example of a SiGe DIACESD protection structure 600 in accordance with the present invention.

FIGS. 7-21 are a series of cross-sectional views illustrating an exampleof a method of forming the SiGe DIAC ESD protection structures 400, 500,and 600 in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 4 shows a cross-sectional view that illustrates an example of aSiGe DIAC ESD protection structure 400 in accordance with the presentinvention. Structure 400 is similar to transistor 300 and, as a result,utilizes the same reference numerals to designate the elements which arecommon to structure 400 and transistor 300.

As shown in FIG. 4, SiGe DIAC ESD protection structure 400 includessemiconductor structure 308 which has a p-substrate 310, and an n+buried layer 312 that touches and lies over p-substrate 310. Inaddition, semiconductor structure 308 includes an n-type collectorregion 314 that touches the top surface of n+ buried layer 312, and anumber of shallow trench isolation regions 318 that extend down from thetop surface of semiconductor structure 308.

As further shown in FIG. 4, structure 400 also includes a pair ofspaced-apart base/emitter structures 410 and 412 that are formed on thetop surface of semiconductor structure 308 to touch n-type collectorregion 314 and a common shallow trench isolation region 318C. Thebase/emitter structures 410 and 412 both have a p-typesingle-crystal-silicon germanium-carbon base region 320 that touches thetop surface of n-type collector region 314, and a p+ polysilicongermanium-carbon base contact region 322 that touches the side ofsingle-crystal-silicon germanium-carbon base region 320.

In addition, a first area 314F of n-type collector region 314 lies belowand touches the p-type single-crystal-silicon germanium-carbon baseregion 320 of base/emitter structure 410, and a second area 314S ofn-type collector region 314 lies below and touches the p-typesingle-crystal-silicon germanium-carbon base region 320 of base/emitterstructure 412. The first and second areas, 314F and 314S, in turn, arelaterally spaced apart by only the common isolation region 318C. Also, aface 320F of p-type single-crystal-silicon germanium-carbon base region320 of base/emitter structure 410, and a face 320S of p-typesingle-crystal-silicon germanium-carbon base region 320 of base/emitterstructure 412 touch isolation region 318C, directly oppose each other,and are substantially parallel.

As further shown in FIG. 4, the base/emitter structures 410 and 412 alsoboth have a silicide layer 324 that touches the top surface of region322, and a metal base contact 326 that touches silicide layer 324. Inaddition, the base/emitter structures 410 and 412 both have an n+polysilicon emitter region 330 that touches the top surface ofsingle-crystal-silicon germanium-carbon base region 320, and an n+emitter region 332 that lies in single-crystal-silicon germanium-carbonbase region 320. The base/emitter structures 410 and 412 bothadditionally have an isolation region 340 that isolates base region 322from emitter region 330, a silicide layer 342 that touches the topsurface of region 330, and a metal emitter contact 344 that touchessilicide layer 342.

Further, the metal base contact 326 and the metal emitter contact 344 ofbase/emitter structure 410 are connected together, and to a pad 414.Similarly, the metal base contact 326 and the metal emitter contact 344of base/emitter structure 412 are connected together, and to a pad 416.

During normal operation, when pad 414 is connected to ground and pad 416is connected to a positive voltage less than the breakdown voltage, thepositive voltage is also placed on polysilicon germanium base contactregion 322 of base/emitter structure 412, and thereby onsingle-crystal-silicon germanium-carbon base region 320 of base/emitterstructure 412.

The positive voltage on single-crystal-silicon germanium-carbon baseregion 320 forward biases the junction between p-typesingle-crystal-silicon germanium-carbon base region 320 of base/emitterstructure 412 and the n-type collector region 314, thereby causing holesto be injected into n-type collector region 314. The injected holesraise the potential on n-type collector region 314, thereby reversebiasing the junction between n-type collector region 314 and p-typesingle-crystal-silicon germanium-carbon base region 320 of base/emitterstructure 410. The reverse-biased junction blocks charge carriers fromflowing from pad 416 to pad 414.

In response to an ESD event, however, the reverse-biased junctionbetween n-type collector region 314 and p-type single-crystal-silicongermanium-carbon base region 320 of base/emitter structure 410 breaksdown due to avalanche multiplication. The breakdown of the junctioncauses holes to be injected into p-type single-crystal-silicongermanium-carbon base region 320 of base/emitter structure 410, andelectrons to be injected into n-type collector region 314. The holesinjected into p-type single-crystal-silicon germanium-carbon base region320 of base/emitter structure 410 then flow over to polysilicongermanium-carbon base region 322 to be collected by metal base contact326 of base/emitter structure 410.

In addition, the flow of holes increases the potential of p-typesingle-crystal-silicon germanium-carbon base region 320 of base/emitterstructure 410 in the region that lies adjacent to n+ region 332, therebyforward biasing the junction between p-type single-crystal-silicongermanium-carbon base region 320 and n+ emitter region 332 ofbase/emitter structure 410.

As a result, p-type single-crystal-silicon germanium-carbon base region320 also injects holes into n+ emitter region 332, and n+ emitter region332 injects electrons into p-type single-crystal-silicongermanium-carbon base region 320 of base/emitter structure 410. Some ofthe electrons injected to base region 320 drift over and are then sweptinto n-type collector region 314 across the broken down junction. Theelectrons swept into n-type collector region 314 are injected intop-type base region 320 of base/emitter structure 412 across theforward-biased junction.

In addition, due to the symmetry between the base/emitter structures 410and 412, the polarities of the pads 414 and 416 can be reversed inresponse to an ESD event. In this case, the above described operationremains the same, but reversed between the base/emitter structures 410and 412.

FIG. 5 shows a cross-sectional view illustrating an example of a SiGeDIAC ESD protection structure 500 in accordance with the presentinvention. Structure 500 is similar to structure 400 and, as a result,utilizes the same reference numerals to designate the elements that arecommon to both structures.

As shown in FIG. 5, structure 500 differs from structure 400 in thatstructure 500 includes a p-well 510 that is formed in semiconductorstructure 308 to extend down from the top surface of structure 308 totouch p-substrate 310. As further shown in FIG. 5,single-crystal-silicon germanium-carbon base 320 of base/emitterstructure 412 contacts p-well 510.

During normal operation, when pad 414 is connected to ground and pad 416is connected to a positive voltage less than the breakdown voltage, thepositive voltage is also placed on polysilicon germanium-carbon basecontact region 322 of base/emitter structure 412, and thereby onsingle-crystal-silicon germanium-carbon base region 320 of base/emitterstructure 412, p-well 510, and p-substrate 310.

The positive voltage on p-well 510 and p-substrate 310 forward biasesthe junction between p-well 510/substrate 310 and n-type collectorregion 314/buried layer 312, thereby causing holes to be injected inton-type collector region 314/buried layer 312. The injected holes raisethe potential on n-type collector region 314, thereby reverse biasingthe junction between n-type collector region 314 and p-typesingle-crystal-silicon germanium-carbon base region 320 of base/emitterstructure 410. The reverse-biased junction blocks charge carriers fromflowing from pad 416 to pad 414.

In response to an ESD event, however, the reverse-biased junctionbetween n-type collector region 314 and p-type single-crystal-silicongermanium-carbon base region 320 of base/emitter structure 410 breaksdown due to avalanche multiplication. The breakdown of the junctioncauses holes to be injected into p-type single-crystal-silicongermanium-carbon base region 320 of base/emitter structure 410, andelectrons to be injected into n-type collector region 314. The holesinjected into p-type single-crystal-silicon germanium-carbon base region320 of base/emitter structure 410 then flow over to polysilicongermanium-carbon base region 322 to be collected by metal base contact326 of base/emitter structure 410.

In addition, the flow of holes increases the potential of p-typesingle-crystal-silicon germanium-carbon base region 320 of base/emitterstructure 410 in the region that lies adjacent to n+ region 332, therebyforward biasing the junction between p-type single-crystal-silicongermanium-carbon base region 320 and n+ emitter region 332 ofbase/emitter structure 410.

As a result, p-type single-crystal-silicon germanium-carbon base region320 injects holes into n+ emitter region 332, and n+ emitter region 332injects electrons into p-type single-crystal-silicon germanium-carbonbase region 320 of base/emitter structure 410. Some of the electronsinjected into base region 320 drift over and are then swept into n-typecollector region 314 across the broken down junction. The electronsswept into n-type collector region 314 are injected into p-typesingle-crystal-silicon germanium-carbon base region 320 of base/emitterstructure 412 across the forward-biased junction.

FIG. 6 shows a cross-sectional view illustrating an example of a SiGeDIAC ESD protection structure 600 in accordance with the presentinvention. Structure 600 is similar to structure 500 and, as a result,utilizes the same reference numerals to designate the elements that arecommon to both structures.

As shown in FIG. 6, structure 600 differs from structure 500 in thatstructure 600 includes an n+ region 610 and a p+ region 612 that areformed in p-well 510, and metal contact structures 614 and 616 thattouch n+ region 610 and p+ region 612, respectively, in lieu ofbase/emitter structure 412. Further, metal contact structures 614 and616 are connected together, and to pad 416.

During normal operation, when pad 414 is connected to ground and pad 416is connected to a positive voltage less than the breakdown voltage, thepositive voltage is also placed on p+ region 612, and thereby on p-well510 and p-substrate 310. The positive voltage on p-well 510 andp-substrate 310 forward biases the junction between p-well 510/substrate310 and n-type collector region 314/buried layer 312, thereby causingholes to be injected into n-type collector region 314/buried layer 312.The injected holes raise the potential on n-type collector region 314 ofbase/emitter structure 410, thereby reverse biasing the junction betweenn-type collector region 314 and p-type single-crystal-silicongermanium-carbon base region 320 of base/emitter structure 410. Thereverse-biased junction blocks charge carriers from flowing from pad 416to pad 414.

In response to an ESD event, however, the reverse-biased junctionbetween n-type collector region 314 and p-type single-crystal-silicongermanium-carbon base region 320 of base/emitter structure 410 breaksdown due to avalanche multiplication. The breakdown of the junctioncauses holes to be injected into p-type single-crystal-silicongermanium-carbon base region 320 of base/emitter structure 410, andelectrons to be injected into n-type collector region 314. The holesinjected into p-type single-crystal-silicon germanium-carbon base region320 of base/emitter structure 410 then flow over to polysilicongermanium-carbon base region 322 to be collected by metal base contact326 of base/emitter structure 410.

In addition, the flow of holes increases the potential of p-typesingle-crystal-silicon germanium-carbon base region 320 of base/emitterstructure 410 in the region that lies adjacent to n+ region 332, therebyforward biasing the junction between p-type single-crystal-silicongermanium-carbon base region 320 and n+ emitter region 332 ofbase/emitter structure 410.

As a result, p-type single-crystal-silicon germanium-carbon base region320 also injects holes into n+ emitter region 332, and n+ emitter region332 injects electrons into p-type single-crystal-silicongermanium-carbon base region 320 of base/emitter structure 410. Some ofthe electrons injected into base region 320 drift over and are thenswept into n-type collector region 314 across the broken down junction.The electrons swept into n-type collector region 314 are injected intop-well 510 across the forward-biased junction.

The SiGe DIAC ESD protection structures 400, 500, and 600 can be formedwith only minor modifications to any conventional SiGe HBT process flow.FIGS. 7-21 show a series of cross-sectional views that illustrate anexample of a method of forming the SiGe DIAC ESD protection structures400, 500, and 600 in accordance with the present invention.

As shown in FIG. 7, the method utilizes a conventionally-formedsemiconductor structure 708 that includes a p-substrate 710, and an n+buried layer 712 that touches and lies over p-substrate 710. Inaddition, semiconductor structure 708 includes an n-type collectorregion 714 that extends down from the top surface of semiconductorstructure 708 to touch the top surface of n+ buried layer 712, and anumber of shallow trench isolation regions 718 that extend down from thetop surface of semiconductor structure 708.

As further shown in FIG. 7, the method begins by epitaxially growing ap-type silicon germanium carbon layer 720 on semiconductor structure708. The regions above n-type collector region 714 grow as a singlecrystal silicon germanium carbon layer 722, while the regions above theshallow trench isolation regions 718 grow as a polysilicon germaniumcarbon layer 724. While silicon germanium carbon is illustrated in thisexample, other semiconductor materials can alternately be used.

As shown in FIG. 8, after silicon germanium carbon layer 720 has beengrown, a mask 726 is formed and patterned on silicon germanium carbonlayer 720. Following this, a p-type dopant, such as boron, is implantedto form p+ regions 730 in n-type collector region 714, and p+ regions732 in silicon germanium carbon layer 720. Alternately, as shown in FIG.9, mask 726 can be patterned such that no p+ region 730 is formed, andthe p+ regions 732 are formed in polysilicon germanium carbon layer 724.Following the implant, mask 726 is removed.

Once mask 726 has been removed, as shown in FIG. 10, a layer ofisolation material 734, such as a layer of oxide and an overlying layerof nitride, is formed on silicon germanium carbon layer 720. Next, amask 736 is formed and patterned on isolation layer 734. Following this,the exposed regions of isolation layer 734 are etched away. Mask 736 isthen removed.

As shown in FIG. 11, the etch forms an isolation region 740 and anisolation region 742. Next, as shown in FIG. 12, a layer of n+polysilicon 744 is formed on silicon germanium carbon layer 720 and theisolation regions 740 and 742. After polysilicon layer 744 has beenformed, a mask 746 is formed and patterned on polysilicon layer 744.Following this, as shown in FIG. 13, the exposed regions of polysiliconlayer 744 are etched away to form an emitter region 750 that touchesisolation region 740 and silicon germanium carbon layer 720, and anemitter region 752 that touches isolation region 742 and silicongermanium carbon layer 720. In addition, both emitter regions 750 and752 lie over a common isolation region 718C. Following the etch, mask746 is removed.

As shown in FIG. 14, after the formation of the emitter regions 750 and752, a layer of silicide 754 is formed on the exposed regions of silicongermanium carbon layer 720 and the emitter regions 750 and 752.Following this, a mask 756 is formed and patterned on the isolationregions 740 and 742, and silicide layer 754.

Next, as shown in FIG. 15, the exposed regions of silicide layer 754 andthe underlying regions of silicon germanium carbon layer 720 are etchedaway. The etch forms a base/emitter structure 760 and a base/emitterstructure 762 that both touch common isolation region 718C. Mask 756 isthen removed.

Once mask 756 has been removed, as shown in FIG. 16, a layer isinsulation material 764 is formed on the exposed regions of theisolation regions 718, silicon germanium carbon layer 720, and thebase/emitter structures 760 and 762. Next, a mask 766 is formed andpatterned on insulation layer 764. Following this, the exposed regionsof insulation layer 764 are etched to form openings 770 that expose thesilicide layers 754. Mask 766 is then removed.

Next, as shown in FIG. 17, a layer of conductive material 772 isdeposited on insulation layer 764 to fill up the openings 770. Followingthis, a mask 774 is formed and patterned on conductive layer 772. Aftermask 774 has been formed, the exposed regions of conductive layer 772are etched until removed. Mask 774 is then removed.

As shown in FIG. 18, the etch forms a first contact 780, a secondcontact 782, a first metal trace 784 that touches the first and secondcontacts 780 and 782. The etch also forms a third contact 790, a fourthcontact 792, and a second metal trace 794 that touches the third andfourth contacts 790 and 792. First metal trace 784 electrically connectsthe polysilicon germanium carbon layer 720 of base/emitter structure 760to the emitter region 750 of base/emitter structure 760. Second metaltrace 794 electrically connects the polysilicon germanium carbon layer720 of base/emitter structure 762 to the emitter region 752 ofbase/emitter structure 762.

The method then continues with conventional steps to form metalinterconnect structures that include pads 796 and 798 that are connectedto metal traces 784 and 794, respectively. The described method issimilar to the process for forming a SiGe HBT as taught in U.S. Pat. No.7,202,136 issued on Apr. 10, 2007, which is hereby incorporated byreference.

SiGe DIAC ESD protection structure 500 can be formed in the same mannerthat structure 400 was formed, except that before silicon germaniumcarbon layer 720 is grown, a mask 1410 is formed on semiconductorstructure 708 as shown in FIG. 19. Following this, the exposed regionsof semiconductor structure 708 are implanted with a p-type material,such as boron, to form a p-well 1412 that extends down and touchesp-substrate 710. Mask 1410 is then removed, and the method continues asdescribed above with the growth of silicon germanium carbon layer 720.

SiGe DIAC ESD protection structure 600 can be formed in the same mannerthat structure 500 was formed, except that after p-well 1412 has beenformed and mask 1410 has been removed, a mask 1510 is formed andpatterned on the top surface of semiconductor structure 708 as shown inFIG. 20. Following this, the exposed regions of semiconductor structure708 are implanted with a p-type material, such as boron, to form a p+region 1512 in the top surface of p-well 1412. Mask 1510 is thenremoved.

As shown in FIG. 21, after mask 1510 has been removed, a mask 1514 isformed and patterned on the top surface of semiconductor structure 708.Following this, the exposed regions of semiconductor structure 708 areimplanted with an n-type material, such as phosphorous, to form an n+region 1516 in the top surface of p-well 1412. Mask 1514 is then removedand the process continues as above, except that a base/emitter structureis not formed over p-well 1412.

It should be understood that the above descriptions are examples of thepresent invention, and that various alternatives of the inventiondescribed herein may be employed in practicing the invention. Thus, itis intended that the following claims define the scope of the inventionand that structures and methods within the scope of these claims andtheir equivalents be covered thereby.

1. A method of forming an electrostatic discharge (ESD) protectiondevice on a structure, the structure having a semiconductor region of afirst conductivity type and an isolation region located in thesemiconductor region, the method comprising: forming a layer ofsemiconductor material of a second conductivity type on a top surface ofthe semiconductor region and a top surface of the isolation region;forming a first region of the first conductivity type on a top surfaceof the layer of semiconductor material; removing a portion of the layerof semiconductor material to expose the isolation region and form asecond region of the second conductivity type that touches the isolationregion; and electrically connecting the first and second regionstogether and to a first pad, a third region of the first conductivitytype being formed on a top surface of the layer of semiconductormaterial when the first region is formed, the first and third regionsbeing spaced apart.
 2. The method of claim 1 wherein removing theportion of the layer of semiconductor material also forms a fourthregion of the second conductivity type, the second and fourth regionsbeing spaced apart by a region that lies directly vertically over theisolation region.
 3. The method of claim 2 and further comprisingelectrically connecting the third and fourth regions together and to asecond pad.
 4. The method of claim 3 wherein: the second region liesover and touches a first portion of the semiconductor region; and thefourth region lies over and touches a second portion of thesemiconductor region, only the isolation region lying laterally betweenthe first portion and the second portion of the semiconductor region. 5.The method of claim 4 wherein the second and fourth regions includesilicon germanium.
 6. The method of claim 4 wherein the second regionincludes a region of single crystal silicon that touches a region ofpolysilicon, and the fourth region includes a region of single crystalsilicon that touches a region of polysilicon.
 7. The method of claim 4wherein: the second region has a first portion of a first dopantconcentration that touches a second portion of a second dopantconcentration that is less than the first dopant concentration of thefirst portion of the second region; and the fourth region has a firstportion of a first dopant concentration that touches a second portion ofa second dopant concentration that is less than the first dopantconcentration of the first portion of the fourth region.
 8. The methodof claim 3 and further comprising forming a well of the secondconductivity type in the semiconductor region before the layer ofsemiconductor material is formed, the layer of semiconductor materialtouching the well when the layer of semiconductor material is formed. 9.The method of claim 8 wherein: the second region lies over and touches aportion of the semiconductor region; and the fourth region lies over andtouches a portion of the well, only the isolation region lying laterallybetween the portion of the semiconductor region and the portion of thewell.
 10. The method of claim 9 wherein the second and fourth regionsinclude silicon germanium.
 11. The method of claim 9 wherein the secondregion includes a region of single crystal silicon that touches a regionof polysilicon, and the fourth region includes a region of singlecrystal silicon that touches a region of polysilicon.
 12. The method ofclaim 9 wherein the second region has a first portion of a first dopantconcentration that touches a second portion of a second dopantconcentration that is less than the first dopant concentration of thefirst portion of the second region; and the fourth region has a firstportion of a first dopant concentration and a second portion of a seconddopant concentration that is less than the first dopant concentration ofthe first portion of the fourth region.
 13. A method of forming anelectrostatic discharge (ESD) protection device on a structure, thestructure having a semiconductor region of a first conductivity type andan isolation region located in the semiconductor region, the methodcomprising: forming a layer of semiconductor material of a secondconductivity type on a top surface of the semiconductor region and a topsurface of the isolation region; forming a first region of the firstconductivity type on a top surface of the layer of semiconductormaterial; removing a portion of the layer of semiconductor material toexpose the isolation region and form a second region of the secondconductivity type that touches the isolation region; electricallyconnecting the first and second regions together and to a first pad; andforming a well of the second conductivity type in the semiconductorregion before the layer of semiconductor material is formed, the layerof semiconductor material touching the well when the layer ofsemiconductor material is formed.
 14. The method of claim 13 and furthercomprising forming a third region of the first conductivity type and afourth region of the second conductivity type in the well before thelayer of semiconductor material is formed, the layer of semiconductormaterial touching the third and fourth regions when the layer ofsemiconductor material is formed.
 15. The method of claim 14 whereinremoving the portion of the layer of semiconductor material removes thelayer of semiconductor from over the third and fourth regions.
 16. Themethod of claim 15 and further comprising electrically connecting thethird and fourth regions to a second pad.
 17. The method of claim 16wherein the second region includes silicon germanium.
 18. The method ofclaim 16 wherein the second region includes a region of single crystalsilicon that touches a region of polysilicon.
 19. The method of claim 16wherein the second region has a first portion of a first dopantconcentration and a second portion of a second dopant concentration thatis less than the first dopant concentration of the first portion of thesecond region.